The present invention refers generally to a plate heat exchanger, in particular a plate heat exchanger in the form of an evaporator, i.e. a plate heat exchanger designed for evaporation of a fluid, i.e. a cooling agent, in a circuit for various applications, such as air conditioning, cooling systems, heat pump systems, etc.
A plate heat exchanger, typically includes a plate package, with a number of first heat exchanger plates and a number of second heat exchanger plates, which are joined to each other and arranged side by side in such a way that a first plate interspace is formed between each pair of adjacent first heat exchanger plates and second heat exchanger plates and a second plate interspace between each pair of adjacent second heat exchanger plates and first heat exchanger plates. The first plate interspaces and the second plate interspaces are separated from each other and provided side by side in an alternating order in the plate package. Substantially each heat exchanger plate has at least a first porthole and a second porthole, wherein the first portholes form a first inlet channel to the first plate interspaces and the second portholes form a first outlet channel from the first plate interspaces.
The fluid supplied to the inlet channel of such a plate heat exchanger for evaporation is usually present both in a gaseous state and a liquid state. This is known as a two-phase evaporator. It is difficult to provide an optimal distribution of the fluid to the different plate interspaces in such a way that e.g. an optimized quantity of fluid is supplied and flows through each plate interspace. In this general prior art plate heat exchanger the fluid is introduced at one end of the first inlet channel, i.e. the first port hole, for further distribution along the first inlet channel and further into each of the individual first plate interspaces. First of all it is very hard to control the flow inside the first inlet channel. There is always a risk of that the energy content of the inserted fluid is too high, whereby a part of the flow supplied to the inlet channel via its inlet port will meet the rear end of the inlet channel and be reflected thereby in the opposite direction. Thereby the flow in the inlet channel is very chaotic and hard to predict and control. Further, the pressure drop of the fluid increases with the distance from the inlet to the first inlet channel, whereby the distribution of fluid between the individual plate interspaces will be affected. Thereby it is hard to optimize the efficiency of the plate heat exchanger. It is also known that the angular flow change that the droplets of the fluid must undergo when entering the individual plate interspaces from the first inlet channel contributes to a mal distribution.
Further, by each heat exchanger plate being provided with port holes, normally at least two, the available heat transfer surface is reduced. Also, the size and the position of the port holes are structural design parameters that must be considered when designing the surface corrugation of the heat exchanger plates in order to optimize the flow along the plate interspaces. The available acting space for flow optimization is thereby restricted.
Documents WO94/14021, WO00/70292 and WO08/000823 disclose examples of a plate heat exchanger wherein the cooling agent is supplied via an insert extending longitudinally inside and along the first inlet channel for distribution of the fluid into the individual plate interspaces.
Generally the efficiency of a plate heat exchanger at part load is a raising issue for the purpose of reducing the energy consumption. By way of example, laboratory scale trials have shown that a cooling system relating to air-conditioning may save 4-10% of its energy consumption just by improved evaporator function at part load for a given brazed plate heat exchanger. Further, an evaporator system is typically only operating at full capacity for 3% of the time, while most evaporators are designed and tuned for a full capacity operation duty. More focus is put on how the evaporator performs at different operation duties instead of being measured at only one typical operation duty. Also, the market applies so called seasonal efficiency standards. The standards may vary between different states and regions. Typically, such standards are based on a consideration including different working loads, whereby most evaporators are designed and tuned in view of a specific standard. However, during normal operation the work load varies greatly and it hardly reflects the fictive conditions used for the standard.